U.S. Pat. No. 3,776,945 teaches a process of depolymerizing poly(ethylene terephthalate) waste to obtain dimethyl terephthalate and ethylene glycol by subdividing the waste into dimensions between 4 and 35 mesh and treating at a temperature of 100.degree. C. to 300.degree. C. and a pressure from 1 to 150 atmospheres with methanol in a quantity that the proportion of methanol to waste is between 1:1 and 10:1 by weight in the presence of acid catalysts.
U.S. Pat. No. 3,321,510 relates to a process of decomposing poly(ethylene terephthalate) by first treating with steam at a temperature of from about 200.degree. C. to 450.degree. C. and then reducing the steam-treated poly(ethylene terephthalate) in the form of a brittle solid product to a powder having a mean particle size of from about 0.0005 to 0.002 millimeters and subsequently atomizing the fine powder with a gaseous substance including inert gas and methanol vapor to form an aerosol which is conducted through a reaction zone at a temperature of 250.degree. C. to 300.degree. C. in the presence of excess methanol vapors.
U.S. Pat. No. 3,037,050 relates to the recovery of terephthalic acid dimethyl ester by treating poly(ethylene terephthalate) in the form of bulky or lumpy solid masses with super-heated methanol vapor in the presence of any suitable esterification catalyst substantially at atmospheric pressure.
U.S. Pat. No. 4,578,502 relates to a procedure for recovering monomeric polycarboxylic acids and polyols from solid scrap polyesters by granulating the scrap resin, slurrying the resin with sufficient solvents such as water or methanol, depolymerizing the slurried resin by the application of heat and pressure for a time sufficient to convert substantially all of the resin into its monomeric components, crystallizing the monomeric polycarboxylic acid present by flash crystallization and recovering the polycarboxylic acid and then the polyol by distillation.
U.S. Pat. No. 4,163,860 relates to a process for converting a bis-(diol) terephthalate to dimethyl terephthalate by interchange in a substantially anhydrous methanol medium in the presence of a magnesium methylate catalyst.
U.S. Pat. No. 3,701,741 relates to a method of recovering substantially pure poly(ethylene terephthalate) from scrap poly(ethylene terephthalate) contaminated with impurities by dissolving the contaminated material at elevated temperatures and super-atmospheric pressure in a volatile solvent. This patent does not relate to the recovery of the monomeric ingredients that comprise the polymer.
U.S. Pat. No. 3,488,298 relates to a process for recovering dimethyl terephthalate and ethylene glycol from poly(ethylene terephthalate) scrap by forming a mixture comprising the poly(ethylene terephthalate) scrap, catalyst and methanol, heating the mixture to approach equilibrium, treating the partially hydrolyzed mixture with an excess of phosphorus-containing compound, heating the treated mixture to fractionate the constituents and recovering methanol, ethylene glycol and dimethyl terephthalate.
U.S. Pat. No. 5,051,528 claims a process for recovering ethylene glycol and dimethyl terephthalate from poly(ethylene terephthalate) polyesters which comprises dissolving scrap polyester in oligomers of ethylene glycol and terephthalic acid or dimethyl terephthalate, passing superheated methanol through the solution and recovering the ethylene glycol and dimethylterephthalate.
Scrap, off-class production, and consumer waste poly(ethylene terephthalate) (PET) and its copolymers may be depolymerized using methanol to its monomeric components for reuse in PET production. However, there is a need to provide consistently high-purity raw materials for PET production to maintain polymer properties such as color, melting point, impact strength, and melt viscosity. Typically the PET waste stream contains polymer produced from different resin producers who use various polymer modifiers to impart specific PET properties. During the methanolysis depolymerization process, these modifiers are present as impurities which need to be removed from the main reaction products of dimethyl terephthalate (DMT) and ethylene glycol (EG). Examples of these impurities include cyclohexane dimethanol (CHDM), dimethyl isophthalate (DMI), diethylene glycol (DEG), and triethylene glycol (TEG).
European Patent Application Publication No. 484963 A2 describes a process for the preparation of dimethyl terephthalate from polymers of terephthalic acid and a glycol which comprises treating said polymers in a reaction zone with methanol vapors at a temperature above 230.degree. C. and at a pressure below about 15 atmospheres, continuously removing vapors of methanol, dimethyl terephthalate, and the glycol from the reaction zone, said vapors containing at least about 3 moles of methanol for every mole of dimethyl terephthalate, separating methanol from said vapors, and separating dimethyl terephthalate from the vapors.
The present invention as described below provides a process yielding high purity DMT which can be used for further polymer production. Simple product separation by conventional distillation is not feasible due to the relatively close boiling points of several of these impurities to DMT. Further, the existence of low-boiling azeotropes of the glycols with DMT result in unacceptable DMT yield losses when employing distillation alone.
Previous patents dealing with the depolymerization of PET have addressed the reaction mechanisms for the depolymerization reaction. Purification of DMT as related to its separation from EG is addressed in U.S. Pat. No. 4,578,502, yet no mention is made of removal of CHDM, DMI, or other glycols. Distillation is unsuitable for the removal of CHDM, DEG, and TEG due to the presence of close boiling points and minimum boiling azeotropes. The separation of DMT by melt crystallization from products from oxidation of p-xylene and subsequent esterification with methanol is taught by U.S. Pat. No. 4,683,034. However, in this system, no CHDM impurities are present which, at levels well below 0.5 wt %, will cause off-specification PET production.